Tc 9-524 fundamentals of machine tools

8-15

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TC 9-524
Indexing
Indexing is the process of evenly dividing the circumference
of a circular workpiece into equally spaced divisions, such as
in cutting gear teeth, cutting splines, milling grooves in
reamers and taps, and spacing holes on a circle. The index
head of the indexing fixture is used for this purpose.
Index Head
The index head of the indexing fixture (Figure 8-19)
contains an indexing mechanism which is used to control the
rotation of the index head spindle to space or divide a
workpiece accurately. A simple indexing mechanism consists
of a 40-tooth worm wheel fastened to the index head spindle,
a single-cut worm, a crank for turning the wormshaft, and an
index plate and sector. Since there are 40 teeth in the worm
wheel, one turn of the index crank causes the worm, and
consequently, the index head spindle to make 1/40 of a turn;
Whenever possible, the workpiece should be clamped in
so 40 turns of the index crank revolve the spindle one full
the center of the vise jaws. However, when necessary to mill
a short workpiece which must be held at the end of the vise, a
spacing block of the same thickness as the piece should be
placed at the opposite end of the jaws. This will avoid strain
on the movable jaw and prevent the piece from slipping. If
the workpiece is so thin that it is impossible to let it extend
over the top of the vise, hold down straps are generally used.
See Figure 8-24. These straps are hardened pieces of steel,
having one vertical side tapered to form an angle of about
92° with the bottom side and the other vertical side tapered to
a narrow edge. By means of these tapered surfaces, the
turn.
Index Plate
The indexing plate (Figure 8-25) is a round plate with a
series of six or more circles of equally spaced holes; the
index pin on the crank can be inserted in any hole in any
circle. With the interchangeable plates regularly furnished
with most index heads, the spacing necessary for most gears,
boltheads, milling cutters, splines, and so forth can be
obtained. The following sets of plates are standard
workpiece is forced downward into the parallels, holding
equipment:
them firmly and leaving the top of the workpiece fully
exposed to the milling cutter.